Supercritical Solvents (CO2, H2O).

Prof. Attilio Citterio

Dipartimento CMIC “Giulio Natta”

https://iscamapweb.chem.polimi.it/citterio/it/education/course-topics/

School of Industrial and Information Engineering

Course 096125 (095857)

Introduction to Green and Sustainable Chemistry

Attilio Citterio

2

Supercritical Fluids.

• Above 31.1 °C and 73 atm (Tcr and

Pcr) carbon dioxide behaves as a

supercritical fluid and shows

properties of both a liquid and a gas.

• It fills the container, like a gas, and

dissolves substances like a liquid.

• Once the critical temperature and pressure have

been reached the two distinct phases of liquid

and gas are no longer visible. The meniscus can

no longer be seen. One homogenous phase

called the "supercritical fluid" phase occurs.

•

SOLID

GAS

LIQUID SUPERCRITICAL

FLUID

Triple Point

Critical Point

•

Pre

ssure

(bar)

Temperature (K)303

74

(31.1ºC)(-56.6ºC)

8

1

(-78.2ºC)

With an increase in temperature

the meniscus begins to diminish.

The liquid density falls due to expansion and the gas

density rises as more of the substance evaporates.

Attilio Citterio

3Comparison of Physical and Transport

Properties of Gases, Liquids and SCFs.

Gas SCF Liquid

Density (g·cm-3) 10-3 0.1-1 1

Viscosity (g·cm-1·s-1) 10-4 10-3-10-4 10-2

Diff. Coeff. (cm2·s-1) 10-2-10-1 10-4-10-3 10-5

Attilio Citterio

4

Common Supercritical Fluids.

A supercritical fluid (SCF) is any substance at a temperature and

pressure above its critical values. Critical temperature is the temperature

above which a pure gas cannot be liquefied regardless pressure values.

They have properties intermediate between gases and liquids, which can

be controlled by both temperature and pressure. Typical critical data are:

Carbon dioxide Tc = 31.1 °C Pc = 73.8 bar

Fluoroform Tc = 25.9 °C Pc = 48.2 bar

Water Tc = 374.0 °C Pc = 220.6 bar

Ammonia Tc = 132.4 °C Pc = 113.2 bar

Ethane Tc = 32.2 °C Pc = 48.7 bar

Methanol Tc = 239.5 °C Pc = 8.08 MPa

CO2 is unpolar high volumes and pressure are necessary to liquefy (high energetic costs). Co-solvents are added to increase the polarity.

Water becomes SC at higher temperatures and pressures than CO2.

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5

Why Use sc-Fluids?

• Increased mass transport

• Gases are totally miscible

• No surface tension

• Excellent for infusion and extraction

• Inert and non-toxic

• Inexpensive fluids

• Environmentally compatible

• Solvent is tuneable with pressure

A.A. Clifford in “Supercritical Fluids”. Eds. E. Kiran and J.M.H. Levet

Sengers, kluver Academic Publishers. 1994, 449-479.

P.G. Jessop, T. Ikaraya and R. Noyori. Science, 1995, 269, 1065: see also:

P.G. Jessop. T. Ikariya and R. Noyori, Nature, 1994, 368, 231.

D.M. Eaton, K.D. Barle, C.M. Rayner and A.A. Clifford, .Journal of High

Resolution Chromatography, 1993, 16, 66.

Attilio Citterio

6

Carbon Dioxide (CO2).

• Similar advantages to water: Natural, cheap,

Plentiful (0.04% of atmosphere and now rising!)

Available in >99.9% pure form, €90/$110 per 25 kg.

By-product of brewing, ammonia synthesis, combustion

Non-flammable

TLV = 5000 ppm

• Supply chain and related technology already established;

• Non-toxic and properties well understood

asphyxiant at high concentrations (excludes oxygen from lungs)

• Easily removed and recycled, and can be disposed of with no net increase in global CO2 ;

Simple product isolation by evaporation, to 100% dryness.

• No solvent effluent, renewable;

• Potential for product processing (extraction, particle formation, chromatography, etc.).

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7

Pressure-Temperature Phase Diagram for CO2.

0

50

100

150

200

250

300

350

400

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Pressure

(bar)

Temperature (°C)

Melting line

Liquid

Boiling line

CP

Supercritical

fluid

Pc = 73.8 bar

Tc = 31.06oC

Solid

Gas

Attilio Citterio

Pure CO2 PTV Diagram and Projections.8

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9

Other Advantages of scCO2.

• Carbon dioxide is a non polar molecule since the

dipoles of the two bonds cancel one another.

• High compressibility

Large change in solvent properties for relatively small change in pressure – infinite

range of solvent properties available (low viscosity and zero surface tension)

Ability to tune solvent to favour a particular reaction pathway simply by optimising

temperature or pressure (tuneable density)

• Cosolvents can further modify solvent properties (“Tunable” properties)

• High diffusivity offers potential for increase reaction rates.

• Potential for homogeneous catalytic processes.

High solubility of light gases, some catalysts and substrates; bring all together in

single homogeneous phase (hydrocarbons and derivatives with less than 20 carbon

atoms, but not big molecules, i.e. oils, waxes, fats, polymers, proteins, sugars)

• Inert to oxidation; resistant to reduction

Excellent medium for oxidation and reduction reactions.

O C O

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10

De

nsity (

g·m

l-1)

Hild

ebra

nd P

ara

mete

r (cal c

m-3)

0.5

310 K = 37 °C

320 K = 47 °C

330 K = 57 °C

Change of Solvent Power of CO2 from Pressure.

10050 150 200 2500

500

1000

Pc Pressure (bar)

310K

320K

330K

8

6

4

2

Pentane

CyclohexaneCCl4

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11

Problems using scCO2.

Moderate pressures required

Standard HPLC apparatus used in lab, reactors made of stainless

steel, many commercially available.

Can be expensive for large scale work.

Weak solvent

Relatively non-polar, but high quadrupole moment. Use of co-solvents

(MeOH, MeCN, THF, toluene)

CO2 exhibits a low dielectric constant, low polarizability/volume

Simple modification of reagents to improve solubility.

Energy considerations

Compression of CO2 requires energy

Energy consumption reduced minimal decompression and recycling.

CO2 is a Lewis acid - reacts in the presence of good nucleophiles

Often reversible (acid behaviour in H2O), can be exploited synthetically

Attilio Citterio

12Current Consumer Related Applications of

CO2 Technology.

• Carbonated drinks

• Leavening agents in baking

• Solvent extraction• Decaffeinating coffee and tea

• Extracting bitterness from hops to make beer

• Defatting cocoa powder

• Extracting spices and aromatic plants

• Surface coating

• Fumigation (1% in air will eliminate pests in greenhouses)

• Degreasing and dry cleaning applications

• Welding

• pH control (e.g. effluent streams, pulp and paper mills)

• Refrigeration (dry ice, mechanical systems)

• Fire extinguishers

• Life jackets

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13

Can sc-Fluids be Scaled up?

One of the biggest chemical reactions (polyethylene production) is carried out under SC conditions

Natural products such as Caffeine and hops extract are obtained using scCO2

Spray painting process use scCO2 to reduce VOC emissions (>’80%).

Known are also some continuous synthesis which use sc-fluids with capacity of up to 1000 tonnes per year.

But Also:

Textile dyeing

157 nm photoresist processing

Self assembly

Nanoparticles

http://www.thomas-swan.co.uk/

Attilio Citterio

Decaffeination Process.14

SOAKING green coffee beans in water doubles their size, allowing the caffeine to dissolve into water inside the bean.

CAFFEINE REMOVAL occurs in an extraction vessel. Caffeine diffuses into this supercritical carbon dioxide, along with some water.

DECAFFEINATED BEANS at the bottom of the vessel are removed, dried and roasted.

RECOVERY of dissolved caffeine occurs in an absorption chamber. A shower of water droplets leaches the caffeine out of the supercritical carbon dioxide.

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15

Particle Formation.

• Two main complementary techniques:

RESS – Rapid expansion of supercritical solution

SAS – Solvent/AntiSolvent Precipitation

• Allow for the processing of a wide range of materials into solid phases with

useful properties and morphologies, e.g. pharmaceuticals, protein

macroparticles, explosives.

J.W. Tom, G.B. Lim, P.G. Debenedetti and R.K. Prud’homme, in Supercritical Fluid Engineering Science -Fundamentals and Applications, F. Kiran, J.F. Brennecke Eds., ACS Symposium Series 514, 1993, chapter 11.

P.G. Debenedetti, Supercritical Fluids - Fundamentals for Application, Eds. E Kiran and J.M.H. Levelt Sengers, 1994, Kluwer Academic Publishers, 1994, pp. 719-729.

Co-solvent

Drug

Encapsulant

Carbon dioxide

Carbon dioxideP T

Particle are formed

due to decreasing

solubility

Dissolution

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16

Dry Cleaning with scCO2.

Current methods use perchloroethylene

1.5 Million tons used per year (North America)

Hazardous air pollutant and suspected carcinogen

Contaminates drinking water supplies

Contributes to photochemical smog

<5% recycled

Requires heating to remove solvent residues

Characteristic odour

New process use liquid CO2

Requires surfactant (to be recycled)

Phase Diagram for CO2

Cl Cl

ClCl

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17

Dry Cleaning with scCO2.

• No unpleasant odors

• No heat required for drying – energy

efficient and kinder on clothes

• Possible tax credits and reduced

regulatory monitoring

• Utilizes same CO2 used by food and

drink distributors

• Related technology also used for

degreasing (e.g. metal parts),

semiconductor photoresist removal

and spin coating.

• Is a relevant byproduct of biorefinery

industry.

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18

Surfactants.

• A molecule that contains a polar portion and a non polar portion.

• A surfactant can interact with both polar and non polar molecules.

• A surfactant increases the solubility of the otherwise insoluble

substances.

• In water, surfactant molecules tend to cluster into a spherical

geometry

non polar ends on the inside of the sphere

polar ends on the outside

• These clusters are called micelles.polar

apolar

Scheme of

a micelle

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19

Micelle Structure of a Surfactant.

polar solvent

Non-polar solute

non polar end of surfactant

polar end of surfactant

polar

end

polar

end

polar end

polar end

polar endpolar end

polar end

polar end

polar end

polar end

Attilio Citterio

20

Natural and Synthetic Surfactants.

Natural

• Biodegradable

• Commonly soft

• Expensive

e.g.:

• esters of fatty acids

• ethoxylate alcohols

• alcohol ether sulfates

• esters of sucrose

Synthetic

• Frequently not very bio-

degradable

• Economic

• Wide variety/activity

e.g.:

• alkyl benzene sulfonates

• ethoxylate alcohols

• alkyl phenol ethoxylates

• quaternary ammonium salt

Attilio Citterio

21

Surfactants for LCO2 or scCO2.

• Must have both CO2-philic (CO2 loving) and CO2 -phobic functionality.

• In 1994, Joseph M. DeSimone of the University of Nord Carolina State

University published his discovery that polyacrylate polymers having

perfluorinated residues are soluble in liquid or supercritical CO2

present surfactant properties if they contain aromatic groups

(copolymers).

* C C *

H

C

H

H O

O CH2CF

2CF

2CF

2CF

2CF

2CF

2CF

3

n

CO2 phylic

Fragment

C C

HH

H

m

C C *

H

C

H

H O

O CH2CF

2CF

2CF

2CF

2CF

2CF

2CF

3

n

*

CO2 phobic

Fragment

J.L. Kendall, D.A. Canelas, J.L. Young and J.M. DeSimone,

Chem. Rev., 1999, 99, 2663.

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22

Solvent Carbon dioxide

Chain segment “CO2-phylic”

Chain segment “CO2-phobic”

Micelle Structure for a CO2 Surfactant.

Attilio Citterio

Supercritical CO2 Surfactants.23

Surfactant Type Solubility

(wt.%)

Emulsion

Morphology

γ Range

CO2 H2O (Stability in h) Max.

Wo

(mN/m)

(5 sec – 54 min)

CF3O(CF2CF2O)7-15OCF2COOH >3 <0.1 W/C 13.9 1.6-3.4

HOOCCF2O(CF2CF2O)2-7OCF2COOH ~2 - W/C 8.5 -

K.P. Johnston et al., J. Dispersion Sci. and Tech., 2002, 23: p. 81.

Attilio Citterio

24

Other Industrial Uses of scCO2.

Union Carbide Technology

Solution for surface coating

Replaces 40-90% of VOCs with CO2.

• Coatings on aerospace parts

• Adhesion promoters on plastics

Food Industry

• Chocolate biscuits

• Extraction of natural materials (Essential oils and fragrances)

• Additives

CO2 Refrigeration

scCO2 SCF chromatography.

Chem. Eng. News, June 14 1999, 77 (24), 13.

R.S. Oakes, A.A. Clifford and C.M. Rayner, J. Chem. Soc., Perkin Trans. 1, 2001, 917.

Attilio Citterio

25

Technologies for Coating by scCO2.

SCF

Solute

air

Rapid Expansion of Supercritical Solutions (RESS)

Gas Anti-Solvent (GAS) Precipitation with a Compressed

Anti-solvent (PCA)

SCF SCFSCF

Liquid

SoluteLiquid

Solute

SCF

Rich

Liquid

Attilio Citterio

26

Chemical Synthesis in scCO2.

Extremely important polymers

Dupont $40M development facility

Previous methods used CFCs

Easy polymer isolation and drying, and minimal waste

Chem. Eng. News, April 26 1999, 77 (17), 10

• Fluoropolymer (e.g. Teflon) synthesis:

**

F F

F FF

FF

F

n

polymerization

Attilio Citterio

27

Reactions in scCO2.

Homogeneous Catalytic Reactions in Supercritical CO2 :

Tumas, Los Alamos

National Laboratory

Asymmetric Catalytic Hydrogenation of Enamides

Catalytic copolymerization of CO2 with Epoxides

O

RO

O

RO

O O

R

O

CO2

n

Catalyst+ +

RCO2Me

NHAc

RCO2Me

NHAc

Catalyst

H 2 /SC CO

2

Attilio Citterio

28

Lab scCO2 Reactors.

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29

Lab Equipment Design with scCO2.

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30

Factors influencing extraction efficiency

Temperature Pressure Solubility

Structure and particle size of raw material

Mass of CO2 per mass of raw material

Supercritical CO2 Extraction.

Very soluble Sparingly soluble Insoluble

non-polar and moderately Chlorophyll, waxes Sugars, proteins

polar molecules and carotenoids Tannins, amino

<500 MW acids, pesticides

Examples: Triterpenoids, Oleic acid, dodecanol Waxes

alkaloids, lipids <C22

Attilio Citterio

31

Liquid CO2 Extraction with Entrainers.

Very small additions of entrainer modify the extraction characteristics

of CO2

Alcohols (including water), ketones and hydrocarbons are most

commonly used

Entrainers modify the polarity and solvating properties of liquid CO2

Some compounds behave as surfactant agents.

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32

Liquid CO2 Extraction Circuit.

Extr

acto

r

LL e

xtr

acto

r

dryer

concentrator

Bean

Product

Bean in

CO2 recycle

CO2

makeup

Water/

caffeine

Aqueous

Caffeine

byproduct

Water in

water

CO2/caffeine

Fla

sh

Attilio Citterio

33

CO2 tank

Extract

CO2

Extract

Pump

Heat exchanger

Reducing valve

Heat

exchangers

SeparatorsChiller

Extraction columns

Supercritical CO2 Extraction Circuit.

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34

Industrial Extraction with CO2.

Decaffeination of Tea and Coffee - SCO2

Extraction of Hops - LCO2 and SCO2

Defatting of cocoa powder - SCO2

Extraction of oil seeds - SCO2

Extraction of spices and aromatic plants - LCO2 and SCO2

Complete removal of fat

(<0.5%), no loss of polyphenols,

no solvent residues

Cocoa defatting (2009)

54,0%

12,0%

0,5%0%

10%

20%

30%

40%

50%

60%

grasso totale pannello pressato sgrassato con CO2

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35

Extraction of Lipids and Aroma with CO2.

Triglycerides have limited solubility in LCO2

ScCO2 almost exclusively used for fat and oil extraction

CO2 extraction reduces post extraction processing

Oils have lower aroma, colour, free fatty acids and peroxide value

Resulting residue can be used for animal feed

Extraction with LCO2 produces products with an aroma which closely

resembles the starting material

Extraction at 10°C or less minimises degradation of labile molecules

and the formation of undesirable ‘off-notes’

A greater proportion of higher molecular weight compounds are

normally found in CO2 extracts when compared to steam distilled oils

Attilio Citterio

36Extraction of Lipids with CO2(Oenothera biennis - Evening primrose).

0

5

10

15

20

25

50 100 200 300

10°C

25°C

55°C

P(bar)

S(g/kg)

Extracts consist of a variety of fatty acids, including the non-

essential ω-6 polyunstaturated fatty acid γ-linolenic acid (GLA), the

essential ω-6 polyunstaturated fatty acid linoleic acid (LA), oleic

acid, palmitic acid, and stearic acid.

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37

CH2Cl2 Ethanol Liquid CO2

Alfa acids 35-45% 30-40% 40-50%

Beta acids 15-20% 10-15% 18-40%

Other soft resins 3-8% 3-8% 5-20%

Hard resins 2-5% 2-10% None

Olio volatile 1-3% 1-2% 2-8%

Fats and Waxes 1-2% Traces 0-5%

Tannins Traces 1-5% None

Chlorophyll <1% Traces None

Inorganic salts <1% 0.5-1% Traces

Residual solvents <1% 0.01-0.1% None

Water Traces 1-5% 1-5%

Typical yield 28% 38% 19%

Liquid CO2 – Selectivity in the Hop Extraction.

(The extraction process maintains integrity of hop components (unchanged from pellet resinous compounds)The extract represents approximately 25% of the original mass of the pellet. .

Attilio Citterio

Hop Extraction with LCO2.38

0

10

20

30

40

50

60

70

0,5 1 2 3 4 5 6

Oil

Alfa acids

Beta acids

Extraction time (h)

*La frazione di composti dell'estratto precipitati con Pb(OAc)2 è detta acidi alfa.

α-acids* (humulones)

β-acids (lupulones)

R = CH2CH(CH3)2 humulone lupulone

CH(CH3)2 cohumulone colupulone

CH(CH3)CH2CH3 adhumulone adlupulone

OHOH

O

O

R

OH

OH O

OOH

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39

Steam distilled % Liquid CO2 %

-farnesene 18.4 14.5

spathulenol <0.1 1.5

-bisabolol oxide B 19.4 4.6

-bisabolone oxide A 12.0 3.1

-bisabolol <0.1 3.7

chamazulene 15.4 1.3

-bisabolol oxide A 9.9 23.5

Dicycloethers (MW 200) 2.7 37.8

Colour Blue Yellow

Matricaria recutita: Composition and Formation

of Chamazulene Oil During Steam Distillation.

Chamazulene

(artifact)

Matricine

pro-chamazuleneO

OHOH

O

O

OCOCH3

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40

Supercritical Water.

Critical point

P ≈ 22 MPa and T = 374o C.

Below critical Point Temperature

Isotherm shows discontinuity.

Especially on intersection with

saturation line.

Above Critical point

Isotherm shows no discontinuity

SCW is in gas like state.

Gives:

Liquid like density

Gas like viscosity.Volume, V

Pre

ss

ure

, P

Critical point

Vaporsaturation

curveLiquid saturation curve

CB A

Liq

uid

phase

Mixture ofliquid and

vapor

D

Attilio Citterio

41

Supercritical Water (2).

Normal Subcritical Supercritical

T / °C 20 100 374 500

P / MPa 0.1 0.1 22.1 50

er 78 30 2 14

pKw 14 12 20 13

A process in water at raised

temperatures and pressures is

called a hydrothermal process.

The concept comes originally

from the geology. The picture on

the left shows a simplified

overview about different

hydrothermal processes and

includes the vapour pressure

curve of the water.ϑ / °C

r / M

Pa

100 200 300 400 500 600 700

20

25

30

35

15

10

5

0

H2→CH4→

← critical

point

"Oil"

H2

C

Chem.

Attilio Citterio

42

Supercritical Water (3).

Attilio Citterio

43Thermal Physical Properties at Critical

Temperature.

• Properties variation as Cp, k,

h and µ are indicated in figure.

• Large properties variation

noticed in pseudo-critical

region.

• The large increase in

convective coefficient is

indicated near pseudo-critical

line

• The steep decrease in µ and k

is noticed.

Water Properties Variation with Temperature

P = 235 bar h

1/P

m

k

cp

Tpc

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44

SC-Water.

Dielectric constant of water

dependent from temperature/

pressure. SC-Water present a

dielectric constant between 2

and 30, similar to nonpolar

solvents, i.e. hexane (e = 1.8),

and polar solvents, i.e. methanol

(e = 32.6).

Therefore, with SC-Water is

possible dissolve organic

compounds insoluble in water at

ambient condition. Salts have

instead low solubility in these

conditions.

DENSITY(g/cm3)

Relative dielectric constant

of organic compounds

- Propane 1.6

- Hexane 1.8

- Heptane 1.9

- CCl4 2.2

- Benzene 2.3

- Acetone 20.7

- Ethanol 24.5

- Methanol 32.6

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45

SC-Water (2).

Water ionic product behaviour Kw=[H+][OH-];

• at 25°C and P = 1 atm, Kw is 1×10-14(mol/l)2.

• At higher temperature and pressure conditions, this value increase. At 34.5 MPa pressure, a maximum value of 1×10-11

(mol/l)2 is reached (minimum –log Kw) near 300°C.

Under these conditions, we have [H+] = 3×10-6 mol/l, with an

increase of about 30 times of concentration at room conditions. SC-water can be a potential acid catalyst. T

200 300 400 500 6000

10

12

14

16

18

20

22

24

26

28

-lo

g K

w-log Kw = 11

13.8MPa

34.5MPa

69MPa

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46

Black Smokers & Supercritical Water.

• Located just south of the Equator, at

the southern end of the mid-Atlantic

Ridge

• 3 kilometers beneath the surface of

the ocean

• Temperatures measured between

407°C – 464°C by remotely

operated submersibles

• Immense pressure & temperature

combine to create an anomaly that

has both liquid and vapor qualities

• Provide essential nutrients to locally

adapted organisms, microbes, &

phytoplankton.

Attilio Citterio

47Waste Treatment by Oxidation in

Supercritical Water.

• Technology able to

complete decomposition

of all types of

recalcitrant compounds

in short time, producing

CO2, H2O and inorganic

salts.

• Main problems arise from

corrosion phenomena

associated to higher

acidity of water in SC

conditions.

Water Oxygen

Factory

Reactor forSupercriticalWater oxidation

Reaction conditionsT = 400-600°CP = 220-250 atm.

Hazardous waste

substances

Difficult-to-decompose

waste substances

Difficult-to-decompose organic substances,hazardous organic substances

Water, carbon dioxide, inorganic salts

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48

Oxidations in SC-Water.

Equipment for oxidation in supercritical water